Splendid insulation
Page 19
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THE AE GROUP has developed w cal design" of insulated piston adiabatic engine.
Details of it were revealed at last month's Society of Automotive Engineers' congress in Detroit where it attracted interest from engine designers from around the world.
The AE insulated piston has two air gaps, one over the centre of the piston crown and one between an annulus of steel/aluminium roll bonded material and the piston crown periphery.
Both air gaps on the piston are sealed completely by electron beam welding of the roll bonded material, to prevent oil and carbon entering them and affecting insulation. As the accompanying table shows, when tested in a 5.8 litre (354 cuin) diesel engine, the insulated piston gave an exhaust gas temperature some 30 deg C higher than a standard piston, and top ring and crown body temperature were reduced.
AE has also tested its new design of piston in a single cyctinder direct injection diesel engine on a thermal endurance cycle of full power at 2,000 rpm for 25 minutes, followed by idling with full water cooling for five minutes.
After 50 hours at a brake mean effective pressure of 7 bar (102 psi) followed by 30 hours at a brake mean effective pressure of 10 bar (145 psi) the piston was undamaged, says AE.
Evidence of scaling on the crown piece suggested that an operating temperature of over 600 deg C had been reached.
It is likely that piston insulation will soon give a diesel en-gine a thermal efficiency of better than 50 per cent. The best thermal efficiency of any current production engine is about 42 per cent.